Formation of buried color filters in a back side illuminated image sensor using an etching-stop layer

ABSTRACT

A semiconductor image sensor includes a substrate having a first side and a second side that is opposite the first side. An interconnect structure is disposed over the first side of the substrate. A plurality of radiation-sensing regions is located in the substrate. The radiation-sensing regions are configured to sense radiation that enters the substrate from the second side. A buffer layer is disposed over the second side of the substrate. A plurality of elements is disposed over the buffer layer. The elements and the buffer layer have different material compositions. A plurality of light-blocking structures is disposed over the plurality of elements, respectively. The radiation-sensing regions are respectively aligned with a plurality of openings defined by the light-blocking structures, the elements, and the buffer layer.

PRIORITY

The present application is a divisional application of U.S. patentapplication Ser. No. 14/307,781, filed Jun. 18, 2014, entitled“FORMATION OF BURIED COLOR FILTERS IN A BACK SIDE ILLUMINATED IMAGESENSOR USING AN ETCHING-STOP LAYER”, issued as U.S. Pat. No. 9,553,118on Jan. 24, 2017, the entire disclosure of which is incorporated hereinby reference.

BACKGROUND

Semiconductor image sensors are used to sense radiation such as light.Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) andcharge-coupled device (CCD) sensors are widely used in variousapplications such as digital still camera or mobile phone cameraapplications. These devices utilize an array of pixels (which mayinclude photodiodes and transistors) in a substrate to absorb (i.e.,sense) radiation that is projected toward the substrate and convert thesensed radiation into electrical signals.

A back side illuminated (BSI) image sensor device is one type of imagesensor device. These BSI image sensor devices are operable to detectlight from the backside. Compared to front side illuminated (FSI) imagesensor devices, BSI image sensor devices have improved performance,especially under low light conditions. However, traditional methods offabricating BSI image sensor devices may still lead to certainshortcomings for BSI image sensor devices. For example, the fabricationof traditional BSI image sensors generally forms a color filter array ona flat surface above a light-blocking metal grid. However, thedisposition of the color filter array above the metal grid leads to alonger optical path for the light before it can be detected by a desiredpixel. The disposition of the color filter array above the metal gridalso requires accurate alignment between the color filter array and themetal grid, as any misalignment may cause undesirable cross-talk betweenadjacent pixels.

Hence, while existing BSI image sensor devices have been generallyadequate for their intended purposes, they have not been entirelysatisfactory in every aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1-26 are simplified fragmentary cross-sectional side views of aportion of an image sensor device at various stages of fabrication inaccordance with some embodiments.

FIGS. 27-28 are flowcharts each illustrating a method of fabricating animage sensor device in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

FIGS. 1-26 are simplified diagrammatic fragmentary sectional side viewsa BSI image sensor device 30 at various stages of fabrication accordingto aspects of the present disclosure. The image sensor device 30includes an array or grid of pixels for sensing and recording anintensity of radiation (such as light) directed toward a back-side ofthe image sensor device 30. The image sensor device 30 may include acharge-coupled device (CCD), complementary metal oxide semiconductor(CMOS) image sensor (CIS), an active-pixel sensor (APS), or apassive-pixel sensor. The image sensor device 30 further includesadditional circuitry and input/outputs that are provided adjacent to thegrid of pixels for providing an operation environment for the pixels andfor supporting external communication with the pixels. It is understoodthat FIGS. 2 to 6 have been simplified for a better understanding of theinventive concepts of the present disclosure and may not be drawn toscale.

With reference to FIG. 1, the image sensor device 30 includes a devicesubstrate 32. In the illustrated embodiment, the device substrate 32contains a silicon material doped with a p-type dopant such as boron(for example a p-type substrate). Alternatively, the device substrate 32could contain another suitable semiconductor material. For example, thedevice substrate 32 may include silicon that is doped with an n-typedopant such as phosphorus or arsenic (an n-type substrate). The devicesubstrate 32 could also contain other elementary semiconductors such asgermanium and diamond. The device substrate 32 could optionally includea compound semiconductor and/or an alloy semiconductor. Further, thedevice substrate 32 could include an epitaxial layer (epi layer), may bestrained for performance enhancement, and may include asilicon-on-insulator (SOI) structure.

The device substrate 32 has a front side (also referred to as a frontsurface) 34 and a back side (also referred to as a back surface) 36. Thedevice substrate 32 also has an initial thickness 38 that is in a rangefrom about 100 microns (um) to about 3000 um. In the present embodiment,the initial thickness 38 is in a range from about 500 um to about 1000um.

Radiation-sensing regions—for example, pixels 40, 41, and 42—are formedin the device substrate 32. The pixels 40-42 are configured to senseradiation (or radiation waves), such as an incident light 43, that isprojected toward device substrate 32 from the back side 36. The light 43would enter the device substrate 32 through the back side 36 (or theback surface) and be detected by one or more of the pixels 40-42. Thepixels 40-42 each include a photodiode in the present embodiment. Inother embodiments, the pixels 40-42 may include pinned layerphotodiodes, photogates, reset transistors, source follower transistors,and transfer transistors. The pixels 40-42 may also be referred to asradiation-detection devices or light-sensors.

The pixels 40-42 may be varied from one another to have differentjunction depths, thicknesses, widths, and so forth. For the sake ofsimplicity, only three pixels 40-42 are illustrated in FIG. 1, but it isunderstood that any number of pixels may be implemented in the devicesubstrate 32. In the embodiment shown, the pixels 40-42 are formed byperforming an implantation process 46 on the device substrate 32 fromthe front side 34. The implantation process 46 includes doping thedevice substrate 32 with a p-type dopant such as boron. In analternative embodiment, the implantation process 46 may include dopingthe device substrate 32 with an n-type dopant such as phosphorus orarsenic. In other embodiments, the pixels 40-42 may also be formed by adiffusion process.

The pixels 40-42 are separated from one another by a plurality of gapsin the device substrate 32. For example, a gap 45 separates the pixel 40from an adjacent pixel to its left (not illustrated), a gap 46 separatesthe pixels 40-41, and a gap 47 separates the pixels 41-42. Of course, itis understood that the gaps 45-47 are not voids or open spaces in thedevice substrate 32, but they may be regions of the device substrate 32(either a semiconductor material or a dielectric isolation element) thatare located between the adjacent pixels 40-42.

Still referring to FIG. 1, the pixels 40-42 are formed in a region ofthe image sensor device 30 referred to as a pixel region 52 (or apixel-array region). In addition to the pixel region 52, the imagesensor 30 may also include a periphery region 54 and a bonding padregion 56. The dashed lines in FIG. 1 designate the approximateboundaries between the regions 52, 54, and 56, though it is understoodthat these regions 52, 54, and 56 are not drawn in scale herein and mayextend vertically above and below the device substrate 32.

The periphery region 54 includes devices 60 and 61 that need to be keptoptically dark. For example, the device 60 in the present embodiment maybe a digital device, such as an application-specific integrated circuit(ASIC) device or a system-on-chip (SOC) device. The device 61 may be areference pixel that is used to establish a baseline of an intensity oflight for the image sensor device 30.

The bonding pad region 56 includes a region where one or more bondingpads (not illustrated herein) of the image sensor device 30 will beformed in a later processing stage, so that electrical connectionsbetween the image sensor device 30 and external devices may beestablished. Among other things, the bonding pad region 56 may containan isolation structure, such as a shallow trench isolation (STI) 58. TheSTI 58 partially extends into the periphery region 54. One function ofthe STI 58 is that it helps insulate the silicon of the device substrate32 from a conductive pad to be formed in the bonding pad region 56,which will be discussed below in more detail.

Although not illustrated herein for reasons of simplicity, it isunderstood that the image sensor 30 may also include a scribe lineregion. The scribe line region includes a region that separates onesemiconductor die (for example, a semiconductor die that includes thebonding pad region 56, the periphery region 54, and the pixel region 52)from an adjacent semiconductor die (not illustrated). The scribe lineregion is cut therethrough in a later fabrication process to separateadjacent dies before the dies are packaged and sold as integratedcircuit chips. The scribe line region is cut in such a way that thesemiconductor devices in each die are not damaged.

Referring now to FIG. 2, an interconnect structure 65 is formed over thefront side 34 of the device substrate 2. The interconnect structure 65includes a plurality of patterned dielectric layers and conductivelayers that provide interconnections (e.g., wiring) between the variousdoped features, circuitry, and input/output of the image sensor device30. The interconnect structure 65 includes an interlayer dielectric(ILD) and a multilayer interconnect (MLI) structure. The MLI structureincludes contacts, vias and metal lines. For purposes of illustration, aplurality of conductive lines 66 and vias/contacts 68 are shown in FIG.2, it being understood that the conductive lines 66 and vias/contacts 68illustrated are merely exemplary, and the actual positioning andconfiguration of the conductive lines 66 and vias/contacts 68 may varydepending on design needs.

The MLI structure may include conductive materials such as aluminum,aluminum/silicon/copper alloy, titanium, titanium nitride, tungsten,polysilicon, metal silicide, or combinations thereof, being referred toas aluminum interconnects. Aluminum interconnects may be formed by aprocess including physical vapor deposition (PVD) (or sputtering),chemical vapor deposition (CVD), atomic layer deposition (ALD), orcombinations thereof. Other manufacturing techniques to form thealuminum interconnect may include photolithography processing andetching to pattern the conductive materials for vertical connection (forexample, vias/contacts 68) and horizontal connection (for example,conductive lines 66). Alternatively, a copper multilayer interconnectmay be used to form the metal patterns. The copper interconnectstructure may include copper, copper alloy, titanium, titanium nitride,tantalum, tantalum nitride, tungsten, polysilicon, metal silicide, orcombinations thereof. The copper interconnect structure may be formed bya technique including CVD, sputtering, plating, or other suitableprocesses.

Still referring to FIG. 2, a buffer layer 70 is formed over the frontside 34 of the interconnect structure 80. In the present embodiment, thebuffer layer 70 includes a dielectric material such as silicon oxide(SiO₂). Alternatively, the buffer layer 70 may optionally includesilicon nitride (SiN). The buffer layer 70 may be formed by CVD, PVD, orother suitable techniques. The buffer layer 70 is planarized to form asmooth surface by a CMP process.

Thereafter, a carrier substrate 80 is bonded with the device substrate40 through the buffer layer 100 and the interconnect structure 65, sothat processing of the back side 36 of the device substrate 32 can beperformed. The carrier substrate 80 in the present embodiment is similarto the device substrate 32 and includes a silicon material.Alternatively, the carrier substrate 80 may include a glass substrate oranother suitable material. The carrier substrate 80 may be bonded to thedevice substrate 32 by molecular forces a technique known as directbonding or optical fusion bonding or by other bonding techniques knownin the art, such as metal diffusion or anodic bonding.

Among other things, the buffer layer 70 provides electrical isolationbetween the device substrate 32 and the carrier substrate 80. Thecarrier substrate 80 provides protection for the various features formedon the front side 34 of the device substrate 32, such as the pixels40-42 formed therein. The carrier substrate 80 also provides mechanicalstrength and support for processing of the back side 36 of the devicesubstrate 32 as discussed below. After bonding, the device substrate 32and the carrier substrate 80 may optionally be annealed to enhancebonding strength.

Referring now to FIG. 3, after the carrier substrate 80 is bonded to thedevice substrate 32, a thinning process 100 is then performed to thinthe device substrate 32 from the backside 36. The thinning process 100may include a mechanical grinding process and a chemical thinningprocess. A substantial amount of substrate material may be first removedfrom the device substrate 32 during the mechanical grinding process.Afterwards, the chemical thinning process may apply an etching chemicalto the back side 36 of the device substrate 32 to further thin thedevice substrate 32 to a thickness 110, which is on the order of a fewmicrons. In some embodiments, the thickness 110 is greater than about 1um but less than about 3 um. It is also understood that the particularthicknesses disclosed in the present disclosure are mere examples andthat other thicknesses may be implemented depending on the type ofapplication and design requirements of the image sensor device 30.

Referring now to FIG. 4, an anti-reflective coating (ARC) layer 130 isformed over the back side 36 of the device substrate 32. In someembodiments, the ARC layer 130 contains a high-k material. In otherembodiments, the ARC layer 130 may contain another suitableanti-reflective material, for example SiCN, SiN, HfO, Al₂O₃, Ta₂O₅ orZrO. A buffer layer 140 is formed over the ARC layer 130. In someembodiments, the buffer layer 140 contains silicon oxide. In otherembodiments, the buffer layer 140 may contain another suitable material,for example SiCN, SiN, HfO, Al₂, TaO or ZrO. The ARC layer 130 and thebuffer layer 140 may each be formed via one or more deposition processesknown in the art.

Referring now to FIG. 5, a layer 150 is formed over the buffer layer 140on the back side 36. The layer 150 has a material composition that isdifferent from the buffer layer 140. In some embodiments, a sufficientlyhigh etching selectivity exists between the layer 150 and the bufferlayer 140. In other words, the buffer layer 140 and the layer 150 havesubstantially different etching rates, such that the etching process maybe performed to remove one of the layers 140 and 150 without affectingthe other. In some embodiments where the buffer layer 140 containssilicon oxide, the layer 150 may contain silicon nitride (SiN), siliconcarbide (SiC), silicon oxynitride (SiON), titanium nitride (TiN), oreven a suitable metal or metal compound material such as tungsten (W),aluminum copper (AlCu), copper (Cu), etc. The layer 150 is formed tohave a thickness 155. In some embodiments, the thickness 155 is in arange from about 100 angstroms to about 1500 angstroms. This thicknessrange is configured such that the layer 150 may adequately perform itsfunction as an etching-stop layer in subsequent processes, as discussedin more detail below.

Referring now to FIG. 6, a light-blocking layer 170 (also referred to asa light-reflective layer or radiation-blocking layer) is formed over thelayer 150 on the back side 36. The light-blocking layer 170 may beformed by a suitable deposition process known in the art. In variousembodiments, the light-blocking layer 170 may contain a metal material,such as aluminum. In other embodiments, different types oflight-blocking or light-reflective materials may be used to implementthe light-blocking layer 170. The light-blocking layer 170 is formed tohave a thickness 175. In some embodiments, to ensure that thelight-blocking layer 170 can sufficiently block light, the thickness 175is formed to be in a range from about 500 angstroms to about 5000angstroms.

Referring now to FIG. 7, the light-blocking layer 170 is patterned intoa plurality of light-blocking structures 180-183. The patterning of thelight-blocking layer 170 may involve one or more photolithographyprocesses known in the art. As a result, the light blocking structures180-182 are formed in the pixel region 52, and the light-blockingstructure 183 is formed in the periphery region 54. The light-blockingstructures 180-184 may collectively be referred to as a “metal grid.”

The light-blocking structures 180-182 are each vertically aligned with arespective one of the gaps 45-47 that separate adjacent pixels 40-42.For example, the light-blocking structure 180 is vertically aligned withthe gap 45 that separates the pixel 40 from the pixel located to itsleft (not illustrated herein), the light-blocking structure 181 isvertically aligned with the gap 46 that separates the pixels 40 and 41,and the light-blocking structure 182 is vertically aligned with the gap47 that separates the pixels 41 and 42.

In this manner, the light-blocking structures 180-182 reduce cross-talk.In more detail, cross-talk may arise when light targeted for one pixel(e.g., pixel 41) spreads to one or more neighboring pixels (e.g., pixels40 or 42). Cross-talk will negatively affect image sensor performance,such as degradation of spatial resolution, reduction of overall opticalsensitivity, and poor color separation. Therefore, the light-blockingstructures 180-182 are implemented between neighboring pixels, so thatlight that would have incorrectly traveled to an adjacent pixel will beblocked and/or reflected back by the light-blocking structures 180-182,thereby reducing cross-talk. As discussed above, the light-blockingstructures 180-182 are formed to have a thickness range (i.e., thethickness of the light-blocking layer 170) of 500 angstroms to about5000 angstroms, so that the light-blocking structures 180-182 can blocklight effectively.

The light-blocking structure 183 is formed in the periphery region 54.The light-blocking structures 183 may substantially block light fromreaching the digital device 60 or the reference pixel 61 that aresupposed to be kept optically dark. No light-blocking structures areformed in the bonding pad region 56, since the bonding pad region 56will be “opened” subsequently to define a bonding pad.

It can be seen that the light-blocking structures 180-183 and the layer150 therebelow collectively define a plurality of openings (or trenches)190-192. These openings 190-192 are reserved for the formation of acolor filter array. In other words, a plurality of color filters will beformed to fill the openings 190-192, respectively (as discussed below inmore detail with reference to FIG. 15). Since the light-blockingstructures 180-182 are vertically aligned with the gaps 45-47, theopenings 190-192 are vertically aligned with the pixels 40-42,respectively. As such, the color filters to be formed in the openings190-192 would be aligned with the pixels 40-42, respectively.

Referring now to FIG. 8, a capping layer 200 is formed over the layer150 and over the light-blocking structures 180-183 on the back side 36.The capping layer 200 may be formed via one or more suitable depositionprocesses known in the art. One or more polishing processes may also beperformed to ensure that the capping layer 200 has a flat or planarizedsurface. A material composition for the capping layer 200 is selectedsuch that a high etching selectivity exists between the capping layerand the layer 150. In other words, the capping layer 200 and the layer150 have substantially different etching rates, such that the etchingprocess may be performed to remove one of the layers 200 and 150 withoutaffecting the other. In some embodiments, the capping layer 200 containsa dielectric material that is the same as (or substantially similar to)the buffer layer 140. For example, the capping layer 200 and the bufferlayer 140 may each contain silicon oxide.

Referring now to FIG. 9, the bonding pad region 56 is “opened”. In moredetail, one or more etching processes may be performed to removeportions of the capping layer 200, the layer 150, the buffer layer 140,the ARC layer 130, and the substrate 32 in the bonding pad region 56,until the STI 58 is exposed. Meanwhile, the pixel region 52 and theperiphery region 54 remain substantially un-etched.

Referring now to FIG. 10, the STI 58 in the bonding pad region 56 isremoved, for example by one or more etching processes. However, aportion of the STI 58 still remains in the periphery region 54, sincethe periphery region 54 is not etched. A portion of the interlayerdielectric material is also removed in the bonding pad region 56, forexample by one or more etching processes. The removal of the STI 58 andthe removal of the interlayer dielectric material in the bonding padregion 56 allows one of the conductive lines 66 to be exposed in thebonding pad region 56.

Referring now to FIG. 11, a conductive pad 220 is formed on the exposedsurface of the conductive line 66 in the bonding pad region 56. Theconductive pad 220 may be formed by one or more deposition andpatterning processes. In some embodiments, the conductive pad 220contains aluminum. In other embodiments, the conductive pad 220 maycontain another suitable metal, for example copper. A bonding wire (oranother electrical interconnection element) may be attached to theconductive pad 220 in a later process, and accordingly the conductivepad 220 may also be referred to as a bond pad or a bonding pad. Also,since the conductive pad 220 is formed on the conductive line 66, it iselectrically coupled to the conductive line 66 and the rest of theinterconnect structure 65 through the conductive line 66. In otherwords, electrical connections may be established between external deviceand the image sensor device 30 at least in part through the conductivepad 220.

It is understood that the conductive pad 220 may be formed to be thickeror thinner than the STI 58. In addition, the conductive pad 220 need notnecessarily cover the entire bonding pad region 56, and therefore theconductive pad 220 may be formed to be spaced apart from the STI 58(i.e., away from the periphery region 54).

Referring now to FIG. 12, the capping layer 200 is removed, for examplethrough one or more suitable etching processes. The layer 150 serves asan etching-stop layer during the removal of the capping layer 200. Theetching processes involved in removing the capping layer 200 areconfigured such that a high etching selectivity exists between thecapping layer 200 and the layer 150. For example, the etching rate forthe capping layer 200 is substantially greater (e.g., by factors of 10)than the etching rate for the layer 150, such that the capping layer 200may be removed while causing a negligible impact on the layer 150. Thus,the etching process “stops” at the layer 150, and portions of the layer150 become exposed (by the openings 190-192) after the removal of thecapping layer 200.

Had the layer 150 not been formed, the removal of the capping layer 200may have caused substantial portions of the buffer layer 140 to beremoved as well, since the buffer layer 140 and the capping layer 200may have substantially similar material compositions (e.g., siliconoxide). This is undesirable, since the buffer layer 140 is in theoptical path of incoming light, and the removal thereof may degrade theoptical performance of the image sensor device 30. The presentdisclosure prevents the undesired etching of the buffer layer 150 withthe implementation of the layer 150 as an etching-stop layer herein.

Referring now to FIG. 13, portions of the layer 150 exposed by theopenings 190-192 are removed, for example through one or more etchingprocesses known in the art. Again, the etching processes involved inetching the layer 150 are configured such that a high etchingselectivity exists between the layer 150 and the buffer layer 140therebelow. For example, the etching rate for the layer 150 issubstantially greater (e.g., by factors of 10) than the etching rate forthe buffer layer 140, such that the layer 150 may be removed whilecausing a negligible impact on the buffer layer 140. In other words, thelayer 140 may serve as an etching-stop layer while the layer 150 isetched. Meanwhile, portions of the layer 150 disposed below thelight-blocking structures 180-183 are kept intact, since they areprotected by the light-blocking structures 180-183 during the etching ofthe layer 150. It is understood that the disposition of the portions ofthe layer 150 underneath the light-blocking structures 180-183 is one ofthe unique physical characteristics of the image sensor device 30according to certain embodiments of the present disclosure.

One reason for the removal of the layer 150 within the trenches 190-192is that color filters will be formed in the trenches 190-192, meaninglight will be traveling through these trenches 190-192. The material ofthe layer 150 (e.g., SiN) may be opaque, or at least not as transparentas the buffer layer 140 therebelow. Therefore, if the layer 150 in thetrenches 190-192 is not removed, it will interfere with the reception oflight and degrade the optical performance of the image sensor device 30.On the other hand, the presence of the portions of the layer 150 belowthe light-blocking structures 180-183 does not substantially interferewith the optical performance of the image sensor device 30, as lightshould not propagate through these portions of the layer 150 anywayi.e., light propagating through the layer 150 would have led toundesirable cross-talk between neighboring pixels.

Referring now to FIG. 14, a passivation layer 240 is formed over theback side 36 of the image sensor device 30. The passivation layer 240may be formed by a suitable deposition process known in the art. Thepassivation layer 240 is formed over the conductive pad 220 in thebonding pad region 56, on the sidewalls of the device substrate 32 andthe layers 130-150 in the periphery region 54, and around each of thelight-blocking structures 180-183 and on the exposed surfaces of thelayers 140-150 in the pixel region 52. The passivation layer 240protects the various layers therebelow from elements such as dust,moisture, etc. In some embodiments, the passivation layer 240 contains adielectric material, such as silicon oxide, silicon nitride, siliconoxynitride, etc. In some embodiments, the passivation layer 240 isformed in a conformal manner.

Referring now to FIG. 15, a portion of the passivation layer 240 in thebonding pad region 56 is removed, for example through one or moreetching processes. The partial removal of the passivation layer 240 inthe bonding pad region 56 exposes a portion of the conductive pad 220 inthe bonding pad region 56. Note that the removal of the passivationlayer 240 is performed in a manner such that portions of the passivationlayer 240 (such as portions 240A and 240B) still remain on theconductive pad 220 in the bonding pad region 56 even after the portionof the passivation layer 240 has been removed. This is because there isno reason to completely remove the passivation layer 240 in the bondingpad region 56. For example, the removal of the passivation layer 240(i.e., the “opening” of the passivation layer 240) merely needs to beperformed to ensure that the exposed surface of the conductive pad 220is sufficiently wide to receive a bonding wire. As such, it would haveplaced an unnecessarily high burden on the fabrication processes tocompletely remove the passivation layer 240 in the bonding pad region56. In addition, complete removal of the passivation layer 240 in thebonding pad region 56 may also lead to the erosion or inadvertentremoval of the passivation layer 240 where it is desired, such as on thesidewall surfaces of the device substrate 32, the layers 130-150, and onthe light-blocking structure 183.

For these reasons discussed above, the portions 240A-240B of thepassivation layer will remain on the conductive pad 220, and thepresence of the portions 240A-240B of the passivation layer in thebonding pad region 56 is also one of the unique observablecharacteristics of the present disclosure. Stated differently, thedisposition of the remnant portions 240A-240B of the passivation layerin the bonding pad region 56 is not necessarily intentional ordeliberate, as these remnant portions 240A-240B of the passivation layermay or may not serve any specific functional or structural purposes.Rather, the presence of the remnant portions 240A-240B of thepassivation layer is a byproducts of the unique fabrication process flowof the present disclosure.

A plurality of color filters 250-252 is also formed in the openings190-192, respectively. In some embodiments, the color filters 250-252may contain an organic material and may be formed by one or more coatingand lithography processes. The color filters 250-252 may also beassociated with different colors. For example, the color filter 250 mayallow a red light to pass through but will filter out all the othercolors of light, the color filter 251 may allow a green light to passthrough but will filter out all the other colors of light, and the colorfilter 252 may allow a blue light to pass through but will filter outall the other colors of light.

The color filters 250-252 may be referred to as buried color filters (ora buried color filter array), since they are buried or embedded in theopenings 190-192 defined by the light-blocking structures 180-183,rather than being formed over or above the light-blocking structures180-183. In this manner, the color filters 250-252 are also verticallyaligned with the pixels 40-42, respectively. In other words, thealignment between the color filters 250-252 and the pixels 40-42 isattributed at least in part to the fact that the light-blockingstructures 180-182 are vertically aligned with the gaps 45-47 separatingthe neighboring pixels. As such, it may also be said that the colorfilters 250-252 are “self-aligned” with the pixels 40-42. Again, theself-aligned color filters 250-252 of the present disclosure improve thecross-talk performance of the image sensor device 30. Furthermore, thefact that the color filters 250-252 are now “buried” or “embedded” inthe openings 190-192 also results in shorter optical paths between thecolor filters 250-252 and the pixels 40-42, which improves the receptionof the light in the pixels 40-42.

A bonding wire 265 is also attached to the conductive pad 220 in thebonding pad region 56. The bonding wire 265 may be attached using a wirebonding process known in the art. The wire bonding process may include aball bonding process, in which a portion of the bonding wire 265 ismelted to form a bonding ball 270. In one embodiment, the bonding wire265 and the bonding ball 270 each include gold. In other embodiments,the bonding wire 265 and the bonding ball 270 may include copper oranother suitable metal. The bonding ball 270 has a lateral dimensionthat is smaller than a lateral dimension than the exposed surface of theconductive pad 220.

It is understood that the order of the opening of the passivation layer240 in the bonding pad region 56, the attachment of the bonding wire265, and the formation of the color filters 250-252 is not critical. Forexample, in some embodiments, the passivation layer 240 is first“opened” in the bonding pad region 56 to expose the conductive pad 220,and then the color filters 250-252 are formed. In other embodiments, thecolor filters 250-252 may be formed first, and the passivation layer 240may then be “opened” in the bonding pad region 56 to expose theconductive pad 220 for the attachment of the bonding wire 265.

FIGS. 1-15 illustrate a process flow for forming buried or embeddedcolor filters for a back-side illuminated image sensor according to someembodiments of the present disclosure. FIGS. 16-26 illustrate anotherprocess flow for forming buried or embedded color filters for aback-side illuminated image sensor according to some alternativeembodiments of the present disclosure, as discussed in detail below. Forreasons of consistency and clarity, similar components appearing inFIGS. 16-26 will be labeled the same.

Referring to FIG. 16, the alternative fabrication process flow iscarried out in the same manner as the process flow described above withreference to FIGS. 1-6, up to the point before the layer 150 is formed.In other words, the images sensor device 30 has undergone the formationof the pixels 40-42, the interconnect structure 65, bonding with thecarrier substrate 80, thinning of the back side 36, and the formation ofthe ARC layer 130 and the buffer layer 140. However, rather than formingthe layer 150, the alternative process flow bypasses this step, andforms the light-blocking layer 170 over the buffer layer 140 on the backside 36.

Referring now to FIG. 17, the light-blocking layer 170 is patterned intothe plurality of light-blocking structures 180-183. The light-blockingstructures 180-183 and the buffer layer 140 collectively define theplurality of openings 190-192. Again, the light-blocking structures180-182 are vertically aligned with the gaps 45-47 separating the pixels40-42, respectively, and the openings 190-192 are vertically alignedwith the pixels 40-42, respectively.

Referring now to FIG. 18, a dielectric layer 300 is formed over thelight-blocking structures 180-183 and over the buffer layer 140 on theback side 36. The dielectric layer 300 protects the various layerstherebelow (and the light-blocking structures 180-182) from elementssuch as dust, moisture, etc. Therefore, the dielectric layer 300 mayalso be referred to as a passivation layer. In some embodiments, thedielectric 300 contains silicon oxide. In some embodiments, thedielectric layer 300 is formed in a conformal manner. That is, thedielectric layer 300 has a thickness 305 is substantially uniformthroughout. In some embodiments, the thickness 305 is in a range fromabout 50 angstroms to about 500 angstroms.

Still referring to FIG. 18, a layer 310 is formed over the layer 310 onthe back side 36. The layer 310 has a material composition that isdifferent from the material composition of the dielectric layer 300. Insome embodiments, the layer 310 contains silicon nitride. In alternativeembodiments, the layer 310 may contain silicon carbide, siliconoxynitride, titanium nitride, or even a suitable metal or metal compoundmaterial such as tungsten, aluminum copper, copper, etc. The materialcomposition for the layer 310 is configured such that it (or thedielectric layer 300 below) may function as an etching-stop layer inetching processes to be discussed later. For example, the materialcomposition of the layer 310 is selected such that a sufficiently highetching selectivity may exist between the layers 300 and 310 (andbetween the layer 310 and the layer to be formed thereon). In otherwords, the layers 300 and 310 have substantially different etchingrates, such that an etching process may be performed to remove one ofthe layers 300 and 310 without affecting the other.

In some embodiments, the layer 310 is formed in a conformal manner. Thatis, the layer 310 has a thickness 315 is substantially uniformthroughout. In some embodiments, the thickness 315 is in a range fromabout 100 angstroms to about 1500 angstroms. This thickness range isconfigured such that the layer 310 may adequately perform its functionas an etching-stop layer in subsequent processes, as discussed in moredetail below.

Referring now to FIG. 19, a capping layer 340 is formed over the layer310 on the back side 36. The capping layer 340 may be formed by adeposition process known in the art. The capping layer 340 fills theopenings 190-192. It is understood that a planarization process (e.g., aCMP process) may be performed to planarize the surface of the cappinglayer 340 after it has been deposited.

The material composition of the capping layer 340 is configured suchthat a sufficiently high etching selectivity may exist between thecapping 340 and the layer 310. In other words, the layers 340 and 310have substantially different etching rates, such that an etching processmay be performed to remove one of the layers 340 and 310 withoutaffecting the other. In this manner, the layer 310 may serve as anetching-stop layer when the capping layer 340 is being etched in a laterprocess. In some embodiments, the capping layer contains silicon oxide.Therefore, in embodiments where the layers 300 and 340 each containsilicon oxide, and the layer 310 contains silicon nitride, it may besaid that an ONO structure oxide/nitride/oxide is formed over the backside 36 of the substrate 32 and over the light-blocking structures180-182.

Referring now to FIG. 20, the bonding pad region 56 is “opened.” In moredetail, one or more etching processes may be performed to removeportions of the capping layer 340, the buffer layer 140, the ARC layer130, and the substrate 32 in the bonding pad region 56, until a portionof the STI 58 is exposed. Meanwhile, the pixel region 52 and theperiphery region 54 remain substantially un-etched. In addition,portions of the layers 300 and 310 disposed on the sidewall of thelight-blocking structure 183 also remain substantially un-etched.

Referring now to FIG. 21, the exposed portion of the STI 58 in thebonding pad region 56 is removed, for example by one or more etchingprocesses. However, a portion of the STI 58 still remains in theperiphery region 54, since the periphery region 54 is not etched. Aportion of the STI 58 disposed below the un-etched portions of thelayers 300 and 310 in the bonding pad region 56 is also un-etched. Aportion of the interlayer dielectric material is also removed in thebonding pad region 56, for example by one or more etching processes. Theremoval of the STI 58 and the removal of the interlayer dielectricmaterial in the bonding pad region 56 allows one of the conductive lines66 to be at least partially exposed in the bonding pad region 56.

Referring now to FIG. 22, a conductive pad 350 is formed on the exposedsurface of the exposed conductive line 66 in the bonding pad region 56.The conductive pad 350 may be formed by one or more deposition andpatterning processes. In some embodiments, the conductive pad 350contains aluminum. In other embodiments, the conductive pad 350 maycontain another suitable metal, for example copper. A bonding wire (oranother electrical interconnection element) may be attached to theconductive pad 350 in a later process, and accordingly the conductivepad 350 may also be referred to as a bond pad or a bonding pad. Also,since the conductive pad 220 is formed on the conductive line 66, it iselectrically coupled to the conductive line 66 and the rest of theinterconnect structure 65 through the conductive line 66. In otherwords, electrical connections may be established between external deviceand the image sensor device 30 at least in part through the conductivepad 350.

It is understood that the conductive pad 350 may be formed to be thickeror thinner than the STI 58. In addition, the conductive pad 350 need notnecessarily cover the entire bonding pad region 56, and therefore theconductive pad 350 may be formed to be spaced apart from the STI 58(i.e., away from the periphery region 54).

Referring now to FIG. 23, the capping layer 340 is removed, for examplethrough one or more suitable etching processes. The layer 310 serves asan etching-stop layer during the removal of the capping layer 340. Theetching processes involved in removing the capping layer 340 (as well asthe material compositions of the layers 340 and 310) are configured suchthat a high etching selectivity exists between the capping layer 340 andthe layer 310. For example, the etching rate for the capping layer 340is substantially greater (e.g., by factors of 10) than the etching ratefor the layer 310, such that the capping layer 340 may be removed whilecausing a negligible impact on the layer 310. Thus, the etching process“stops” at the layer 310, and the removal of the capping layer 340exposes the layer 310.

Referring now to FIG. 24, an etching process is performed to removeportions of the layer 310. In the embodiment illustrated in FIG. 24, theetching process is a dry etching process. As a result of the dry etchingprocess, spacers 310A are formed. The spacers 310A are portions of thelayer 310 that are not removed by the etching process (or stateddifferently, remaining portions of the layer 310 after the etchingprocess is performed). The spacers 310A are disposed on portions of thedielectric layer 300 that are formed on the sidewall surfaces of thelight-blocking structures 180-183. Thus, it may be said that the spacers310A are disposed over the sidewalls of the light-blocking structures180-183 as well. However, the spacers 310A are not disposed over topsurfaces of the light-blocking structures 180-183, since the portions ofthe layer 310 disposed over the top surfaces of the light-blockingstructures 180-183 have already been removed by the dry etching process.In some embodiments (for example when the spacers 310A contain siliconnitride or a metal), the spacers 310A are less transparent (or moreopaque) than the layer 300. This may be beneficial as the opaque spacers310A may aid the light-blocking structures 180-183 in blocking orreflecting light in order to reduce cross-talk.

The shape and profile of the spacers 310A resemble spacers that aretypically formed on the sidewalls of a transistor gate in semiconductorfabrication. For example, the spacers 310A have somewhat curved outersurfaces (away from the light-blocking structures 180-183) and arenarrower at the top and wider at the bottom. However, the materialcompositions of these spacers 310A may or may not be the same as theconventional spacers formed on the sidewalls of the transistor gates.The spacers 310A are formed to have a maximum lateral dimension 370. Insome embodiments, the maximum lateral dimension 370 is approximately thesame as the thickness 315 of the layer 310 (FIG. 18), which is in arange from about 100 angstroms to about 1500 angstroms. It is understoodthat the presence of the spacers 310A is one of the unique physicalcharacteristics of the image sensor device 30 according to certainembodiments of the present disclosure.

As discussed above, a high etching selectivity exists between the layers300 and 310. Therefore, the layer 300 serves as an etching-stop layerduring the etching of the layer 310. In other words, the etching of thelayer 310 does not substantially affect the layer 300. Thus, the highetching selectivity between the layers 300 and 310, as well as betweenthe layers 310 and 340, ensures that the capping layer 340 can beeffectively removed without damaging the buffer layer 140 (which is inthe optical path) therebelow. In more detail, the ONO-like structureformed collectively by the layers 300, 310, and 340 allows the layer 310to serve as an etching-stop layer while the capping layer 340 isremoved, and it also allows the layer 300 to serve as an etching-stoplayer while the layer 310 is removed. At the end of these etchingprocesses, the buffer layer 140 is undamaged, and the passivation layer300 is formed in the openings 190-192, which are reserved for theformation of color filters.

Referring now to FIG. 25, a plurality of color filters 250-252 is formedin the openings 190-192, respectively. In some embodiments, the colorfilters 250-252 may contain an organic material and may be formed by oneor more coating and lithography processes. The color filters 250-252 mayalso be associated with different colors. For example, the color filter250 may allow a red light to pass through but will filter out all theother colors of light, the color filter 251 may allow a green light topass through but will filter out all the other colors of light, and thecolor filter 252 may allow a blue light to pass through but will filterout all the other colors of light.

The color filters 250-252 may be referred to as buried color filters (ora buried color filter array), since they are buried or embedded in theopenings 190-192 defined by the light-blocking structures 180-183,rather than being formed over or above the light-blocking structures180-183. In this manner, the color filters 250-252 are also verticallyaligned with the pixels 40-42, respectively. In other words, thealignment between the color filters 250-252 and the pixels 40-42 isattributed at least in part to the fact that the light-blockingstructures 180-182 are vertically aligned with the gaps 45-47 separatingthe neighboring pixels. As such, it may also be said that the colorfilters 250-252 are “self-aligned” with the pixels 40-42. Again, theself-aligned color filters 250-252 of the present disclosure improve thecross-talk performance of the image sensor device 30. Furthermore, thefact that the color filters 250-252 are now “buried” or “embedded” inthe openings 190-192 also results in shorter optical paths between thecolor filters 250-252 and the pixels 40-42, which improves the receptionof the light in the pixels 40-42.

A bonding wire 265 is also attached to the conductive pad 350 in thebonding pad region 56. Since the passivation layer (i.e., layer 300) inthe bonding pad region 56 has already been removed, the conductive pad350 is exposed and is ready to be bonded to the bonding wire 265. Thebonding wire 265 may be attached using a wire bonding process known inthe art. The wire bonding process may include a ball bonding process, inwhich a portion of the bonding wire 265 is melted to form a bonding ball270. In one embodiment, the bonding wire 265 and the bonding ball 270each include gold. In other embodiments, the bonding wire 265 and thebonding ball 270 may include copper or another suitable metal. Thebonding ball 270 has a lateral dimension that is smaller than a lateraldimension than the exposed surface of the conductive pad 350.

It is understood that the order sequence of the formation of the colorfilters 250-252 and the attachment of the bonding wire 265 is notcritical. For example, in some embodiments, the color filters 250-252may be formed before the attachment of the bonding wire 265. In otherembodiments, the bonding wire 265 may be attached before the formationof the color filters 250-252.

The embodiment discussed above uses a dry etching process to remove thelayer 310, which leaves the spacers 310A on the sidewalls of thelight-blocking structures 180-183. It is also understood that, in somealternative embodiments, a wet etching process may be performed toremove the layer 310 instead of the dry etching process. In that case,the layer 310 may be completely removed. In other words, no spacers 310Awould have been formed if the wet etching process was used to remove thelayer 310. The resulting image sensor device 30 associated with this wetetching process is shown in FIG. 26.

Although not specifically illustrated, it is understood that additionalprocesses may be performed to complete the fabrication of the imagesensor device 30. For example, a plurality of micro-lenses may be formedover the color filters 250-252. The micro-lenses help direct and focuslight towards the pixels 40-42 in the substrate 32. The micro-lenses maybe positioned in various arrangements and have various shapes dependingon a refractive index of material used for the micro-lens and distancefrom a sensor surface. In addition, a plurality of testing, dicing, andpackaging processes may also be performed. For reasons of simplicity,these additional structures and/or processes are not specificallyillustrated or discussed in detail herein.

FIG. 27 is a simplified flowchart illustrating a method 500 offabricating an image sensor device according to embodiments of thepresent disclosure. The method 500 includes a step 510 of providing asubstrate that contains a plurality of radiation-sensing regions formedtherein. The substrate has a first side and a second side. Aninterconnect structure may be formed over the first side of thesubstrate. The substrate (or the semiconductor image sensor device) hasa pixel region, a periphery region, and a bonding pad region. Theradiation-sensing regions are formed in the pixel region.

The method 500 includes a step 515 of bonding the first side of thesubstrate to a carrier substrate. The step 520 is performed such thatthe interconnect structure is bonded between the substrate and thecarrier substrate.

The method 500 includes a step 520 of thinning the substrate from thesecond side after the bonding. In some embodiments, the thinning step520 includes one or more chemical and/or mechanical grinding andpolishing processes.

The method 500 includes a step 525 of forming a buffer layer over thesecond side of the substrate after the thinning. In some embodiments,the buffer layer contains silicon oxide;

The method 500 includes a step 530 of forming a first layer over thebuffer layer, the first layer and the buffer layer having differentmaterial compositions. In some embodiments, the first layer containssilicon nitride, silicon carbide, silicon oxynitride, titanium nitride,tungsten, aluminum copper, or copper. In some embodiments, the step 530of forming the first layer is performed such that the first layer has athickness in a range from about 100 angstroms to about 1500 angstroms.

The method 500 includes a step 535 of forming a plurality oflight-reflective structures over the first layer. The light-reflectivestructures and the first layer define a plurality of openings that areeach aligned with a respective one of the pixels.

The method 500 includes a step 540 of forming a second layer over thelight-reflective structures. The second layer fills the openings. Thesecond layer and the first layer have different material compositions.In some embodiments, the second layer contains silicon oxide.

The method 500 includes a step 545 of removing portions of the substratein the bonding pad region. This step may also be referred to as“opening” the bonding pad region.

The method 500 includes a step 550 of forming a bonding pad in thebonding pad region after the bonding pad region has been opened.

The method 500 includes a step 555 of removing the second layer with afirst etching process. The first layer serves as a first etching-stoplayer in the first etching process.

The method 500 includes a step 560 of removing portions of the firstlayer disposed below the openings with a second etching process. Thebuffer layer serves as a second etching-stop layer in the second etchingprocess.

In some embodiments, the first etching process and material compositionsof the first and second layers are configured such that the first andsecond layers have substantially different etching rates in the firstetching process. In some embodiments, the second etching process andmaterial compositions of the first layer and the buffer layer areconfigured such that the first layer and the buffer layer havesubstantially different etching rates in the second etching process.

It is understood that additional process steps may be performed before,during, or after the steps 510-560 discussed above to complete thefabrication of the semiconductor device. For example, an anti reflectivecoating (ARC) layer may be formed between the substrate and the bufferlayer. As another example, after the etching of the first layer, apassivation layer is formed over the light-reflective structures andover the buffer layer. The passivation layer partially fills theopenings. Thereafter, a plurality of color filters may be formed in theopenings. Additional steps may be performed to complete the image sensorfabrication, but they are not specifically discussed herein for reasonsof simplicity.

FIG. 28 is a simplified flowchart illustrating a method 600 offabricating an image sensor device according to embodiments of thepresent disclosure. The method 600 includes a step 610 of providing asubstrate that contains a plurality of radiation-sensing regions formedtherein. The substrate has a first side and a second side. Aninterconnect structure may be formed over the first side of thesubstrate. The substrate (or the semiconductor image sensor device) hasa pixel region, a periphery region, and a bonding pad region. Theradiation-sensing regions are formed in the pixel region.

The method 600 includes a step 615 of bonding the first side of thesubstrate to a carrier substrate. The step 520 is performed such thatthe interconnect structure is bonded between the substrate and thecarrier substrate.

The method 600 includes a step 620 of thinning the substrate from thesecond side after the bonding. In some embodiments, the thinning step520 includes one or more chemical and/or mechanical grinding andpolishing processes.

The method 600 includes a step 625 of forming a plurality oflight-reflective structures over the second side of the substrate afterthe thinning. The light-reflective structures partially define aplurality of openings that are each aligned with a respective one of thepixels.

The method 600 includes a step 630 of coating a first layer on each ofthe light-reflective structures. In some embodiments, the first layercontains silicon oxide.

The method 600 includes a step 635 of coating a second layer on thefirst layer, wherein the second layer and the first layer have differentmaterial compositions. In some embodiments, the second layer containssilicon nitride, silicon carbide, silicon oxynitride, titanium nitride,tungsten, aluminum copper, or copper.

The method 600 includes a step 640 of filling the openings with a thirdlayer. The third layer and the second layer have different materialcompositions. In some embodiments, the third layer contains siliconoxide.

The method 600 includes a step 645 of removing portions of the substratein the bonding pad region.

The method 600 includes a step 650 of thereafter forming a bonding padin the bonding pad region.

The method 600 includes a step 655 of thereafter etching the thirdlayer. The second layer serves as a first etching-stop layer during theetching of the third layer.

The method 600 includes a step 660 of thereafter etching the secondlayer. The first layer serves as a second etching-stop layer during theetching of the second layer. In some embodiments, the etching of thesecond layer is performed using a dry etching process. This forms aplurality of spacers disposed over sidewalk of the light-structures. Thespacers are portions of the second layer that remain after the etchingof the second layer.

It is understood that additional process steps may be performed before,during, or after the steps 610-660 discussed above to complete thefabrication of the semiconductor device. For example, an anti reflectivecoating (ARC) layer and a buffer layer may be formed between thesubstrate and the light-reflective structures. As another example, afterthe etching of the second layer, a plurality of color filters may beformed in the openings. Additional steps may be performed to completethe image sensor fabrication, but they are not specifically discussedherein for reasons of simplicity.

The embodiments of the present disclosure discussed above offeradvantages over existing art, though it is understood that differentembodiments may offer other advantages, not all advantages arenecessarily discussed herein, and that no particular advantage isrequired for all embodiments. In more detail, conventional BSI imagesensor fabrication process flows typically form a metal grid (i.e.,light-blocking or light-reflective structures) in the back side before aconductive bonding pad is formed. The metal grid needs to be capped by acapping layer such as an oxide layer. This capping layer has to beflattened in subsequent processes, and it will be difficult to removethis capping layer over the metal grid before the bonding pad is formed.As a result, color filters have to be coated over a flat surface of thecapping layer. In other words, the conventional fabrication process flowfor BSI image sensors lead to the color filters being formed over andabove the metal grid, rather than being embedded within the trenches oropenings partially defined by the metal grid. As such, the conventionalBSI image sensors have a longer optical path between the color filtersand pixels, as well as gaps between the color filters and the metal gridfrom which light can escape. Moreover, the disposition of the colorfilters over and above the metal grid requires the metal grid to beaccurately aligned with the color filters (or more precisely, the gapsseparating adjacent color filters). For these reasons discussed above,conventional image sensors tend to suffer from cross-talk issues and/orhave degraded performance in terms of optical loss or quantumefficiency.

In contrast, embodiments of the present disclosure facilitate theformation of buried or embedded color filters. In more detail, theembodiments discussed above with reference to FIGS. 1-15 and 27implements an etching-stop layer to allow the oxide capping layer to beremoved without damaging the buffer layer (or other layer in the opticalpath of the image sensor) below. The embodiments discussed above withreference to FIGS. 16-26 and 28 utilizes an ONO-like structure to alsoallow the oxide capping layer to be removed without damaging the bufferlayer (or other layer in the optical path of the image sensor) below.

In each of these two embodiments discussed above, a metal grid is formedthat defines openings that are reserved for the formation of buriedcolor filters. Stated differently, according to the embodiments of thepresent disclosure, the color filters can be formed to be embedded orburied in the openings defined by the metal grid, rather than beingformed on a flat surface above the metal grid. Consequently, the metalgrid can more effectively prevent the light from incorrectly entering anadjacent pixel (since the metal grid is at the same level as the colorfilters), thereby reducing cross-talk. The fact that the color filtersare formed within the openings defined by the metal grid also means thatthe color filters are “self-aligned”, thereby obviating any alignmentconstraints between the metal grid and the color filters. In addition,the shorter optical path between the color filters and the pixelsincreases light reception and enhances quantum efficiency.

One embodiment of the present disclosure pertains to a semiconductorimage sensor device. The image sensor device includes a substrate havinga first side and a second side that is opposite the first side. Aninterconnect structure is disposed over the first side of the substrate.A plurality of radiation-sensing regions is located in the substrate.The radiation-sensing regions are configured to sense radiation thatenters the substrate from the second side. A buffer layer is disposedover the second side of the substrate. A plurality of elements isdisposed over the buffer layer. The elements and the buffer layer havedifferent material compositions. A plurality of light-blockingstructures is disposed over the plurality of elements, respectively. Theradiation-sensing regions are respectively aligned with a plurality ofopenings defined by the light-blocking structures, the elements, and thebuffer layer.

Another embodiment of the present disclosure pertains to a semiconductorimage sensor device. The image sensor device includes a substrate havinga front side and a back side that is opposite the front side. Aninterconnect structure is disposed over the first side of the substrate.A plurality of pixels is located in the substrate. The pixels are eachconfigured to detect light that enters the substrate from the back side.A dielectric layer is disposed over the back side of the substrate. Aplurality of light-reflective structures is disposed over the back sideof the substrate. A plurality of segments is each disposed between thedielectric layer and a respective one of the light-reflectivestructures. The segments each contain a metal material or a dielectricmaterial different from the dielectric layer. A plurality of colorfilters is disposed between the light-reflective structures. Each of thecolor filters is aligned with a respective one of the pixels.

Yet another embodiment of the present disclosure pertains to a method offabricating a semiconductor image sensor device. A substrate isprovided. The substrate comprises a pixel region, a periphery region,and a bonding pad region. The pixel region contains a plurality ofradiation-sensing regions. The first side of the substrate is bonded toa carrier substrate. Thereafter, the substrate is thinned from a secondside opposite the first side. A buffer layer is formed over the secondside of the substrate after the thinning. A first layer is formed overthe buffer layer. The first layer and the buffer layer have differentmaterial compositions. A plurality of light-reflective structures isformed over the first layer. The light-reflective structures and thefirst layer define a plurality of openings that are each aligned with arespective one of the pixels. A second layer is formed over thelight-reflective structures. The second layer fills the openings. Thesecond layer and the first layer have different material compositions.Portions of the substrate in the bonding pad region are removed.Thereafter a bonding pad is formed in the bonding pad region.Thereafter, the second layer is removed with a first etching process.The first layer serves as a first etching-stop layer in the firstetching process. Thereafter, portions of the first layer disposed belowthe openings are removed with a second etching process. The buffer layerserves as a second etching-stop layer in the second etching process.

Another one embodiment of the present disclosure pertains to asemiconductor image sensor device. The image sensor device includes asubstrate having a first side and a second side that is opposite thefirst side. An interconnect structure is disposed over the first side ofthe substrate. A plurality of radiation-sensing regions are located inthe substrate. The radiation-sensing regions are configured to senseradiation that enters the substrate from the second side. A plurality oflight-blocking structures is disposed over the second side of thesubstrate. A passivation layer is coated on top surfaces and sidewallsof each of the light-blocking structures. A plurality of spacers isdisposed on portions of the passivation layer coated on the sidewalls ofthe light-blocking structures.

Another embodiment of the present disclosure pertains to a semiconductorimage sensor device. The image sensor device includes a substrate havinga front side and a back side that is opposite the front side. Aninterconnect structure is disposed over the first side of the substrate.A plurality of pixels is located in the substrate. The pixels are eachconfigured to detect light that enters the substrate from the back side.A plurality of light-reflective structures is disposed over the backside of the substrate. A passivation layer is coated on top surfaces andsidewalls of each of the light-reflective structures. A plurality ofspacers is disposed on portions of the passivation layer coated on thesidewalls of the light-reflective structures but not over the topsurfaces of the light-reflective structures. The spacers and thepassivation layer have material compositions that are configured suchthat the spacers and the passivation layer have substantially differentetching rates. A plurality of color filters is disposed between thelight-reflective structures. The color filters are each aligned with arespective one of the pixels. The color filters are isolated from thelight-reflective structures by the passivation layer and the spacers.

Yet another embodiment of the present disclosure pertains to a method offabricating a semiconductor image sensor device. A substrate isprovided. The substrate comprises a pixel region, a periphery region,and a bonding pad region. The pixel region contains a plurality ofradiation-sensing regions. The first side of the substrate is bonded toa carrier substrate. Thereafter, the substrate is thinned from a secondside opposite the first side. A plurality of light-reflective structuresis formed over the second side of the substrate after the thinning. Thelight-reflective structures partially define a plurality of openingsthat are each aligned with a respective one of the pixels. A first layeris coated on each of the light-reflective structures. A second layer iscoated on the first layer. The second layer and the first layer havedifferent material compositions. The openings are filled with a thirdlayer. The third layer and the second layer have different materialcompositions. Portions of the substrate in the bonding pad region areremoved. Thereafter, a bonding pad is formed in the bonding pad region.Thereafter, the third layer is etched. The second layer serves as afirst etching-stop layer during the etching of the third layer.Thereafter, the second layer is etched. The first layer serves as asecond etching-stop layer during the etching of the second layer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method of fabricating a semiconductor imagesensor device, comprising: forming a first layer over a substrate,wherein the substrate contains a plurality of radiation-sensing regions;forming a light-blocking layer over the first layer; patterning thelight-blocking layer into a grid structure that includes a plurality offirst openings, wherein each of the first openings is aligned with oneof the radiation-sensing regions, respectively; etching the firstopenings into the first layer; forming a passivation layer in the firstopenings, wherein the passivation layer is formed on sidewalls of thefirst layer and the light-blocking layer, and wherein the passivationlayer reduces the plurality of first openings into a plurality of secondopenings; and forming a plurality of color filters in the secondopenings, wherein a respective one of the color filters is formed ineach of the second openings.
 2. The method of claim 1, furthercomprising, before the forming the light-blocking layer: forming theplurality of radiation-sensing regions in the substrate; forming aninterconnect structure over a first side of the substrate; bonding thesubstrate to a carrier wafer via the interconnect structure; andthinning the substrate from a second side opposite the first side afterthe bonding.
 3. The method of claim 1, wherein the forming of thelight-blocking layer comprises forming a metal layer as thelight-blocking layer.
 4. The method of claim 1, wherein the patterningcomprises patterning the light-blocking layer into the grid structure ina pixel region of the semiconductor image sensor device, wherein thesemiconductor image sensor device further includes a bonding pad regiondifferent from the pixel region.
 5. The method of claim 4, thepatterning is performed such that the grid structure is not formed overthe bonding pad region.
 6. The method of claim 4, further comprising,before the forming of the light-blocking layer: forming a buffer layerover the substrate; wherein the forming of the first layer comprisesforming the first layer ove buffer layer, the first layer and the bufferlayer having different material compositions.
 7. The method of claim 6,further comprising: after the patterning but before the forming of thecolor filters: filling the first openings of the grid structure with asecond layer, the second layer and the first layer having differentmaterial compositions; forming a bonding pad in the bonding pad region;removing the second layer with a first etching process, wherein thefirst layer serves as a first etching-stop layer in the first etchingprocess; and removing portions of the first layer disposed below thefirst openings with a second etching process, wherein the buffer layerserves as a second etching-stop layer in the second etching process. 8.The method of claim 7, wherein: the first etching process is configuredto have etching selectivity between the first layer and the secondlayer; and the second etching process is configured to have etchingselectivity between the first layer and the buffer layer.
 9. The methodof claim 8, wherein the buffer layer and the second layer have a samematerial composition.
 10. The method of claim 7, wherein the forming ofthe first layer and the forming of the second layer are performed in amanner such that: the first layer contains silicon nitride, siliconcarbide, silicon oxynitride, titanium nitride, tungsten, aluminumcopper, or copper; and the second layer contains silicon oxide.
 11. Amethod of fabricating a semiconductor image sensor device, comprising:forming a plurality of light-sensing elements in a substrate, thelight-sensing elements being configured to sense light that enters thesubstrate from a back side of the substrate; forming an interconnectstructure over a front side of the substrate; bonding the substrate to acarrier wafer via the interconnect structure; and thinning the substratefrom the back side after the bonding; forming a buffer layer over theback side of the substrate; forming a first dielectric layer over thebuffer layer, the first dielectric layer and the buffer layer havingdifferent material compositions; forming a metal layer over the backside of the substrate; patterning the metal layer into a metal grid, themetal grid including a plurality of openings that is each aligned with arespective one of the light-sensing elements, respectively; filling theopenings of the metal grid with a second dielectric layer, the seconddielectric layer and the first dielectric layer having differentmaterial compositions; removing the second dielectric layer with a firstetching process, wherein the first dielectric layer serves as a firstetching-stop layer in the first etching process; and removing portionsof the first dielectric layer disposed below the openings with a secondetching process, wherein the buffer layer serves as a secondetching-stop layer in the second etching process; and filling each ofthe openings with a respective color filter.
 12. The method of claim 11,further comprising: forming a passivation layer partially in theopenings before the filling.
 13. The method of claim 11, wherein thesemiconductor image sensor device includes a pixel region that containsthe light-sensing elements and a bonding pad region that is free of thelight-sensing elements, and wherein the metal grid is formed over thepixel region but not over the bonding pad region.
 14. The method ofclaim 13, further comprising: after the patterning but before thefilling: forming a bonding pad in the bonding pad region.
 15. The methodof claim 11, wherein the forming of the first dielectric layer and theforming of the second dielectric layer are performed in a manner suchthat: the first dielectric layer contains silicon nitride, siliconcarbide, silicon oxynitride, or titanium nitride; and the seconddielectric layer contains silicon oxide.
 16. A method of fabricating asemiconductor image sensor device, comprising: providing a substratethat includes a pixel region, a periphery region, and a bonding padregion, wherein the pixel region contains a plurality of light-sensingpixels, the substrate having a first side and a second side opposite thefirst side; bonding the first side of the substrate to a carriersubstrate; thinning the substrate from the second side after thebonding; forming a buffer layer over the second side of the substrateafter the thinning; forming a first layer over the buffer layer, thefirst layer and the buffer layer having different material compositions;forming a plurality of light-reflective structures over the first layer,wherein the light-reflective structures and the first layer define aplurality of openings that are each aligned with a respective one of thepixels; forming a second layer over the light-reflective structures, thesecond layer filling the openings, wherein the second layer and thefirst layer have different material compositions; removing portions ofthe substrate in the bonding pad region; thereafter forming a bondingpad in the bonding pad region; thereafter removing the second layer witha first etching process, wherein the first layer serves as a firstetching-stop layer in the first etching process; and thereafter removingportions of the first layer disposed below the openings with a secondetching process, wherein the buffer layer serves as a secondetching-stop layer in the second etching process.
 17. The method ofclaim 16, wherein: the first etching process and material compositionsof the first and second layers are configured such that the first andsecond layers have substantially different etching rates in the firstetching process; and the second etching process and materialcompositions of the first layer and the buffer layer are configured suchthat the first layer and the buffer layer have substantially differentetching rates in the second etching process.
 18. The method of claim 16,further comprising: forming an antireflective coating (ARC) layerbetween the substrate and the buffer layer; after the removing of theportions of the first layer, forming a passivation layer over thelight-reflective structures and over the buffer layer, the passivationlayer partially filling the openings; and thereafter forming a pluralityof color filters in the openings.
 19. The method of claim 16, whereinthe forming of the first layer is performed such that the first layerhas a thickness in a range from about 100 angstroms to about 1500angstroms.
 20. The method of claim 16, wherein the forming of the bufferlayer, the forming of the first layer, and the forming of the secondlayer are performed in a manner such that: the buffer layer containssilicon oxide; the first layer contains silicon nitride, siliconcarbide, silicon oxynitride, titanium nitride, tungsten, aluminumcopper, or copper; and the second layer contains silicon oxide.